1. Field Invention and Related Art Statement
The present invention relates to an apparatus for reproducing digital information out of an optical record medium, and more particularly to an apparatus for reproducing digital information recorded on tracks in an optical record medium as discrete record portions representing bits of the digital information.
In an apparatus for reading digital information recorded on an optical record medium such as an optical disk and an optical card, a signal read out of the optical record medium has to be converted into a bivalent signal. Heretofore, in Japanese Patent Application Publication Kokai Sho 61-16070, there has been disclosed a bivalent circuit for converting the read out signal into the bivalent signal. FIG. 1 shows a block diagram depicting such a known bivalent circuit. As illustrated in FIG. 1, the bivalent signal is constructed such that the reproduced signal read out of the optically readable record medium in which only the bit "1" signal of the digital information is recorded as discrete record portions is converted into the bivalent signal by processing the reproduced signal in a manner explained below. The bivalent circuit comprises differentiating circuit 1, zero cross detector 2, bivalent signal generator 3 and gate signal generator 4.
In the known bivalent circuit shown in FIG. 1, a reproduced signal 10 (FIG. 2A) read out of the optical record medium is supplied to the differentiating circuit 1 to derive a differential signal 11 shown in FIG. 2B. The differential signal 11 is then supplied to the zero cross detector 2 to derive a zero cross signal 14 illustrated in FIG. 2C. As depicted in FIG. 2C, since the reproduced signal contains noise 12, the zero cross signal 14 also includes noise components.
The zero cross detector 2 produces the pulse signal 14 whose state is inverted each time the differential signal 11 crosses the zero potential level 13. The thus produced pulsate zero cross signal 14 is supplied to one input terminal of the bivalent signal generator 3.
The reproduced signal 10 is also supplied to the gate signal generator 4 to derive a gate signal 16 (FIG. 2D) which is in a high level (H) when the reproduced signal is equal to or higher than a reference level 15, and is in a low level (L) when the reproduced signal is lower than the reference level 15. The thus generated gate signal 17 is supplied to the other input terminal of the bivalent signal generator 3.
In the bivalent signal generator 3, a logical operation is performed, i.e., a logical AND between the pulse signal 14 supplied from the zero cross detector 2 and the gate signal 16 supplied from the gate signal generator 4. Therefore it is possible to derive the bivalent signal which represents the digital information recorded in the optical record medium as illustrated in FIG. 2E. That is to say, only peaks of the reproduced signal 10 which exceed the reference level 15 can be detected as the real peaks, so that the noise 12 contained in the reproduced signal 10 can be removed. In this manner the bivalent signal generator 3 produces the bivalent signal 17 illustrated in FIG. 2E.
In another known bivalent circuit, instead of setting the reference level 15 for the reproduced signal 10, a reference level is set for the differential signal 11 to detect whether the recorded information is present or not and, only when the recorded information is detected is the reproduced signal converted into the bivalent signal.
In the known bivalent circuit shown in FIG. 1 in which the reference level is set for the reproduced signal 10, it is very difficult to determine the reference level in the optimum manner, because the level of the reproduced signal changes at respective points on the record medium in accordance with the construction and various properties of the optical record medium, so that the bivalent signal could not be obtained correctly.
In order to remove the above mentioned drawback, the reproduced signal is supplied to one input terminal of a voltage comparator as well as to a low pass filter, and an output signal of the low pass filter is supplied to the other input terminal of the voltage comparator as a threshold level signal. Then an output signal of the voltage comparator is supplied to the bivalent signal generator as the gate signal. However, in such a case, the threshold level signal is delayed with respect to the reproduced signal due to the time constant of the low pass filter, so that when the reproduced signal varies greatly it is impossible to derive the threshold level signal properly and thus there is produced an error in the conversion into the bivalent signal.
In the known bivalent circuit in which the reference level is set for the differential signal, since the noise component contained in the reproduced signal is enhanced, the reproduced signal could not be converted into the bivalent signal faithfully.